It is possible with battery storage devices to receive electrical energy from the grid, if more active power is available from the power generators connected to the grid than is presently required by the consumers connected to the grid. The operator of the battery storage may not, however, indiscriminately accept power from the grid and provide power. Instead, guidelines are set by the network operator which the operator of the battery storage has to satisfy. In particular, guidelines are set by the network operator for the provision of so-called primary control power. The guidelines lead to an agreement between the operator of the battery storage and the network operator, which establishes the primary control power that a battery storage system is guaranteed to be able to remove or provide within a specific time.
The state of charge of the energy storage device, at which the arrangement for receiving electrical energy from a power grid and for discharging electrical energy to the power grid may provide the guaranteed primary control power and also receive the guaranteed primary control power, is designated as the neutral state. In this state of charge, the energy storage device may discharge a sufficiently large discharge current or receive a sufficiently large charging current at a specified nominal voltage of the energy storage device in order to provide the guaranteed primary control power or to receive the same from the grid. The neutral state of the battery storage system is a value that results directly from the guaranteed primary power or—in case the guaranteed primary power is regularly redefined by agreements between the operator of the arrangement for receiving electrical energy from a power grid and for discharging electrical energy to the power grid and the network operator—results from the maximal guaranteeable primary control power.
Accordingly, in the past, battery storage systems were designed such that the neutral state is achieved in such storage systems at a state of charge SOC=50%. At a state of charge SOC=50%, a sufficiently large current could then be received or discharged at the nominal voltage of the battery storage system, in order to receive or discharge the guaranteed primary control power.
The setting of the neutral state to the state of charge SOC=50% necessitated that battery storage in the past was very large and correspondingly expensive.
The performance of the battery storage, required in the past for providing primary control power, and the costs connected thereto have been reasons to search for solutions in recent times, such as those described at the outset.
European patent application 15 187 205 describes one such solution. A similar device is disclosed in document WO 2014/177 175 A1.
The arrangement described in European patent application 15 187 205 for receiving electrical energy from an electrical grid and for discharging electrical energy to the electrical grid solves the problem of reducing the battery storage provided as the energy storage device. This is achieved in that, unlike previously-known arrangements for accepting electrical active power, the energy-converter is made available as well as the battery storage. The provision of electrical active power is carried out, in contrast, only by the battery storage. Since the energy-converter may receive power from the grid, it is possible to reduce the output and thus the size of the battery storage by approximately 50%. The neutral state may then be specified at the maximum charge (SOC=100%) of the battery storage. In the neutral state, the guaranteed output is available to be discharged to the network. It is likewise possible that the guaranteed output may be received from the grid in the neutral state. The power is then used, however, by the energy-converter to convert the energy removed from the network into heat. Storage in the energy storage device is not possible.
If, in such an arrangement according to European patent application 15 187 205, the state of charge is below the neutral state, and active power is to be received from the grid, then initially the battery storage is brought up to the neutral state. If the neutral state is achieved and additional power is to be received, then the received power is used to convert electrical energy into heat. The changeover mechanism is then used for this purpose, which connect the direct current input and output either to the energy-converter or to the energy storage device, thus to the battery storage.
One may use the frequency of the network voltage to detect whether active power is to be received from the grid or whether active power is to be provided to the grid. In the case of an upwards deviation of the network frequency from the nominal network frequency, then power should be received; in the case of a downwards deviation from the nominal network frequency, then power should be provided.
Studies by the applicant have shown that the frequency of the network very often deviates above or below the nominal network frequency by only small amounts. This leads, in the case of a battery storage of an arrangement according to European patent application 15 187 205 in the neutral state, to the fact that the changeover mechanism is often switched in order to connect the direct current input and output of the converter to the energy storage device, whenever the frequency fluctuates around the nominal network frequency. Each time that the nominal network frequency is exceeded in the neutral state, the direct current input and output of the converter are connected to the energy-converter by the changeover mechanism. In addition, the changeover mechanism always connect the direct current input and output of the converter to the energy storage device, whenever the frequency drops below the nominal network frequency, e.g., 50 Hz.
Due to the frequent fluctuation of the network frequency around the nominal network frequency, frequent switching operations also occur.
The frequent switching operations severely load the changeover mechanism such that, if electromechanical switches are used in the changeover mechanism, they must be already replaced after a few months, since the number of switches, for which they were designed, has already been reached.
Instead of electromechanical switches, power electronic circuits may also be used. However, in comparison to electromechanical switches, these are expensive and lead to increased losses.
As a result, a known arrangement of the type listed at the outset may not be operated with the economic advantages that are desired.